Alpha wrong when blitting with BLEND_RGBA_MULT on Mac OS X

I have some code that takes an image, loads it into a surface, and blits an alpha transparent image with BLEND_RGBA_MULT to erase the corners. It works perfectly on Linux and Windows XP, but when I test it on my Mac the corners do not vanish, they just turn yellow. I would guess this means that the colors are mixed up when blitting in this way. The machine in question is a G7 PowerPC Mac. (I don't have access to an Intel Mac) so this could be an endiannness issue.

I am using python 2.5.2 (the python.org build) and pygame 1.8.1release-svn1537
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